The invention relates to an optical scanning device having an improved response characteristic, for use in an apparatus for optically reading or writing information in one or more tracks on a recording medium, which may be, for example, a compact disc (known as a CD), a digital versatile disc (known as a DVD), a CD or DVD which can be written to, or else a magneto-optical recording medium.
Scanning devices for optical recording media are generally known. The construction and operation of an optical scanning apparatus, of a so-called optical pickup, are described in Electronic Components and Applications, Vol. 6, No. 4, 1984, pages 209-215. Such scanning devices have a so-called actuator on which an objective lens is arranged, which is provided for tracking and for focusing the light beam or laser beam on the optical recording medium. In principle, optical scanning devices may be distinguished by the way in which the objective lens is suspended. For example, in the case of a known leaf-spring actuator, the objective lens holder is secured by means of four parallel leaf springs on a frame, cf. EP-A 0 178 077. The disadvantages are that such spring arrangements have an undesirable tendency to oscillate and involve a high level of assembly complexity. Parallel guidance of the objective lens holder is also achieved by an actuator of the jointed or hinged type, as is known, for example, from EP-B 0 563 034. An actuator with parallel guidance has been found to be relatively stable in terms of tilting of the objective lens during deflection, but involves a high level of adjustment complexity since it needs to be aligned exactly in terms of the movement directions that are guided by joints.
Another type of objective lens holder is to use four wires which, as bearing elements, connect the objective lens holder to the actuator baseplate. Socalled wire pick-ups can be produced more costeffectively than optical scanning devices having a leaf spring or joint. However, they have been found to be disadvantageous in comparison with other parallel guides, in terms of tilting of the objective lens and guidance characteristics.
One quality criterion of optical scanning devices is their response characteristic, in which case the term response characteristic means the reaction of the optical scanning device to control signals by means of which the scanning device is deflected for focusing or for tracking, in order to scan a specific point on the recording medium or to follow the movement of the recording medium. The movement of the optical scanning device is intended to follow the applied control signals very accurately. However, as a rule, excitation of a mechanical system leads to sympathetic oscillation in the region of the resonant frequency and to a so-called resonance peak which has a negative effect on the response characteristic and the reaction of the optical scanning device to control signals. Since the resonant frequency is frequently in the range that is audible by the human ear, this effect is furthermore evident in a negative manner as so-called howling. U.S. Pat. No. 4,477,755 has already disclosed the use of a circuit arrangement for electronic compensation of optical and mechanical instabilities in the focusing and tracking control loop in order to avoid mechanical resonances and howling. The circuit arrangement contains a model of the mechanical system, by means of which the reaction of the scanning device to control signals is monitored, and the movement of the scanning device is stopped if a predetermined threshold value is exceeded. For this purpose, the circuit contains a filter and a variable-gain amplifier in order to produce a control signal compensation component at a selected frequency in the region of the frequencies at which mechanical resonances are significant.
The object of the invention is to provide an optical scanning device which, without any electronic compensation means, very largely avoids resonance peaks and has an improved response characteristic.
This object is achieved by the features of the invention specified in independent claims. Advantageous developments of the invention are specified in dependent claims.
One aspect of the invention is to design an optical scanning device in such a manner that it follows applied control signals as uniformly as possible and without any resonance peak occurring.
It has been found that resonance phenomena are significantly influenced by the characteristics of a retaining plate on which the means for bearing the objective lens holder are secured. It has been found that disadvantageous resonance peaks are avoided by the means for bearing the objective lens holder being supported in a flexible manner. On the other hand, the options for selecting an oscillation-damping material for the retaining plate are limited, since, on the one hand, the retaining plate has to bear the weight of the objective lens and the objective lens holder and, on the other hand, it is necessary to ensure that the actuator returns to its original position after it has been deflected. This means that the retaining plate must not be deformed by either force or temperature influences. The contradictory requirements for high flexibility and high strength are satisfied by a retaining plate in which those points on the retaining plate which hold the bearing elements for the objective lens holder are very largely mechanically decoupled. The mechanical decoupling of those points on the retaining plate which hold the bearing elements for the objective lens holder is achieved by a retaining plate which has notches or cutouts in the surface which holds the bearing elements for the objective lens holder. These notches are provided in such a way that, on the one hand, straight edges of the surface which holds the bearing elements are interrupted by notches or openings in the form of slots and, on the other hand, cutouts are provided in the interior of the surface to reduce the effective area between the mounting points for the bearing elements of the objective lens. In one version of the invention, the mounting points are designed as island surfaces which are connected via constrictions to the surface which bears them. This type of configuration for the retaining plate significantly improves the response characteristic of the optical scanning device, and resonance peaks are avoided. The term response characteristic means, in particular, the reaction of the optical scanning device to control signals, as reflected in the frequency response and the relative phase, and in the transfer function of the control signals to the optical scanning system. Resonance peaks are very largely avoided without any electronic compensation means. This effect is rather surprising since it had to be assumed that supporting the bearing elements of the objective lens on a comparatively more highly sprung surface would lead to increased resonance peaks.
The said retaining plate is preferably combined with a second retaining plate to form a retaining system, in which case the second retaining plate makes it possible in an advantageous manner for the first retaining plate to be designed to be thinner, for additional damping to be ensured in the lower frequency range and, furthermore, for the maximum deflection of the actuator to be limited. In the case of one configuration which is provided as a wire pick-up, the wires which are provided as bearing elements are passed through openings in the second retaining plate and are secured on the first retaining plate by, for example, a soldering process. The intermediate space which then remains in the openings for the wire in the second retaining plate is then filled with a filling compound. Silicone is preferably used as the filling compound in order to damp resonance peaks that occur in the low-frequency range. Although it had to be assumed that embedding the wires used as bearing elements in silicone in the second retaining plate would significantly damp resonance peaks, it was found, however, that this is true only to an insufficient extent. The desired response characteristic is achieved only in combination with the first retaining plate according to the invention.